An electrode material used for an electrode of a vacuum interrupter (VI) etc. is required to fulfill the properties of: (1) a great current-interrupting capacity; (2) a high withstand voltage capability; (3) a low contact resistance; (4) a good welding resistance; (5) a lower consumption of contact point; (6) a small interrupting current; (7) an excellent workability; (8) a great mechanical strength; and the like.
A copper (Cu)-chromium (Cr) electrode has the properties of a good current-interrupting capacity, a high withstand voltage capability, a good welding resistance and the like and widely known as a material for a contact point of a vacuum interrupter. The Cu—Cr electrode has been reported that Cr particles having a finer particle diameter are more advantageous in terms of the current-interrupting capacity and the contact resistance (for example, by Non-Patent Document 1).
As a method for producing a Cu—Cr electrode material, two methods, a solid phase sintering method and a infiltration method are generally well known. In the solid phase sintering method, Cu having a good conductivity and Cr having an excellent arc resistance are mixed at a certain ratio, and the mixed powder is press molded and then sintered in a non-oxidizing atmosphere (for example, in a vacuum atmosphere) thereby producing a sintered body. The sintering method has the advantage that the composition between Cu and Cr can freely be selected, but it is higher in gas content than the infiltration method and therefore has a fear of being inferior to the infiltration method in mechanical strength.
On the other hand, in the infiltration method, a Cr powder is press molded (or not molded) and charged into a container and then heated to temperatures of not lower than the melting point of Cu in a non-oxidizing atmosphere (for example, in a vacuum atmosphere) to infiltrate Cu into airspaces defined among Cr particles, thereby producing an electrode. Although the composition ratio between Cu and Cr cannot freely be selected, the infiltration method has the advantage that a material smaller than the solid phase sintering method in gas content and the number of airspaces is obtained, the material being superior to the solid phase sintering method in mechanical strength.
In recent years, conditions for the use of the vacuum interrupter are getting restricted while the application of the vacuum interrupter to a capacitor circuit is increasingly developed. In a capacitor circuit a voltage two or three times the usual one is applied between electrodes, so that it is assumed that the surface of a contact point receives significant damages by arc generated at current-interrupting time or current-starting time thereby causing the reignition of arc easily. For example, when closing electrodes under a state of applying voltage, an electric field between a movable electrode and a fixed electrode is so strengthened as to cause an electrical breakdown before the electrodes are closed. An arc is to be generated at this time, and the surfaces of the contact points of the electrodes cause melting by the heat of the arc. After the electrodes have been closed, the melted portions are reduced in temperature by thermal diffusion so as to be welded. When opening the electrodes, the welded portions are stripped from each other and therefore the surfaces of the contact points are to be damaged. Hence there has been desired an electrode material having better withstand voltage capability and current-interrupting capability than those of the conventional Cu—Cr electrode.
As a method for producing a Cu—Cr based electrode material excellent in electrical characteristics such as withstand voltage capability and current-interrupting capability, there is a method of producing an electrode where a Cr powder for improving the electrical characteristics and a heat resistant element powder (molybdenum (Mo), tungsten (W), niobium (Nb), tantalum (Ta), vanadium (V), zirconium (Zr) etc.) for refining the Cr powder are added to a Cu powder as a base material and then the mixed powder is charged into a mold and press molded and finally obtain a sintered body (Patent Documents 1 and 2, for example).
To be more specific, a heat resistant element is added to a Cu—Cr based electrode material originated from Cr having a particle diameter of 200-300 μm, thereby refining Cr through a microstructure technique. Namely, the method is such as to accelerate the alloying of Cr and the heat resistant element and to increase the deposition of fine Cr—X particles (where X is a heat resistant element) in the interior of the Cu base material structure. As a result, Cr particles having a particle diameter of 20-60 μm is uniformly dispersed in the Cu base material structure, in the form of including the heat resistant element in the interior thereof.
In order to improve an electrode material in electrical characteristic such as current-interrupting capability and withstand voltage capability, it is required that in the Cr base material a content of Cr and that of a heat resistant element are large and that Cr and particles where Cr and the heat resistant element are changed into a solid solution are miniaturized in particle diameter and then uniformly dispersed in the Cu base material.
However, the Cr based particles contained in the electrode material of Patent Document 1 has a particle diameter of 20-60 μm. In order to enhance the electrical characteristics such as current-interrupting capability and withstand voltage capability, these particles are required to be more downsized.
In general, when using a Cr powder having a small average particle diameter as a raw material, it is possible to disperse the refined Cr particles uniformly in the Cu base material. However, a Cr powder having a small average particle diameter is used as a raw material, the oxygen content in the raw material Cr powder is increased, so that the current-interrupting capability of the Cu—Cr based electrode may disadvantageously be reduced.